CODEWITHSUNDEEP
vulkan
Vulkanoid
Vulkanoid

Reputation: 89

Vertex buffer with vertices of different formats

I want to draw a model that's composed of multiple meshes, where each mesh has different vertex formats. Is it possible to put all the various vertices within the same vertex buffer, and to point to the correct offset at vkCmdBindVertexBuffers time? Or must all vertices within a buffer have the same format, thus necessitating multiple vbufs for such a model?

Looking at the manual for vkCmdBindVertexBuffers, it's not clear whether the offset is in bytes or in vertex-strides.

https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/vkCmdBindVertexBuffers.html

Upvotes: 1

Views: 882

Answers (1)

Jherico
Jherico

Reputation: 29240

Your question really breaks down into 3 questions

  1. Does the pOffsets parameter for vkCmdBindVertexBuffers accept bytes or vertex strides?
  2. Can I put more than one vertex format into a vertex buffer?
  3. Should I put more than one vertex format into a vertex buffer?

The short version is

  1. Bytes
  2. Yes
  3. Probably not

Does the offsets parameter for vkCmdBindVertexBuffers accept bytes or vertex strides?

The function signature is

void vkCmdBindVertexBuffers(
    VkCommandBuffer                             commandBuffer,
    uint32_t                                    firstBinding,
    uint32_t                                    bindingCount,
    const VkBuffer*                             pBuffers,
    const VkDeviceSize*                         pOffsets);

Note the VkDeviceSize type for pOffsets. This unambiguously means "bytes", not strides. Any VkDeviceSize means an offset or size in raw memory. Vertex Strides aren't raw memory, they're simply a count, so the type would have to be a uint32_t or uint64_t.

Furthermore there's nothing in that function signature that specifies the vertex format so there would be no way to convert the vertex stride count to actual memory sizes. Remember that unlike OpenGL, Vulkan is not a state machine, so this function doesn't have any "memory" of a rendering pipeline that you might have previously bound.

Can I put more than one vertex format into a vertex buffer?

As a consequence of the above answer, yes. You can put pretty much whatever you want into a vertex buffer, although I believe some hardware will have alignment restrictions on what are valid offsets for vertex buffers, so make sure you check that.

Should I put more than one vertex format into a vertex buffer?

Generally speaking you want to render your scene in as few draw calls as possible, and having lots of arbitrary vertex formats runs counter to that. I would argue that if possible, the only time you want to change vertex formats is when you're switching to a different rendering pass, such as when switching between rendering opaque items to rendering transparent ones.

Instead you should try to make format normalization part of your asset pipeline, taking your source assets and converting them to a single consistent format. If that's not possible, then you could consider doing the normalization at load time. This adds complexity to the loading code, but should drastically reduce the complexity of the rendering code, since you now only have to think in terms of a single vertex format.

Upvotes: 1

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